The study of the natural history of periodontal disease in Sri Lankan tea laborers and Norwegian males is continuing. This past year analyses have been completed both on the reliability and measurement errors associated with the assessment of plaque, calculus, gingival bleeding, and loss of periodontal attachment, and on the longitudinal relationships between gingival restorations and periodontal health. Future studies include the following: (1) Completing and submitting for publication a study of gingival recession in a population who practiced mechanical oral hygiene daily and in a population where oral hygiene never was practiced. (2) Study of the stability of the gingival lesion and conversion of gingivitis to periodontitis which are the critical issues in the quantitative destruction of the periodontium and which can only be studies with some precision in longitudinal materials. (3) Bacteriological and immunological studies of the two populations to relate the presence and absence of selected periodontal pathogens and peripheral blood antibody titers to the rate of periodontal destruction in the two groups. (4) Studies of the patterns and rates of tooth loss over the 25- year period in the Sri Lankan tea laborers.